In industrial practice, the hydrogenation of organic compounds, such as saturated and unsaturated aldehydes or hydroxyaldehydes, is generally carried out continuously, using excess hydrogen. Unreacted hydrogen is recirculated (circulating gas) into the hydrogenation reactor, usually after a part of the hydrogen, laden with inert and ballast substances, has been separated, and the amount separated plus the amount consumed has been replaced by fresh gas.
The overall process conventionally proceeds as set forth below, variations or adaptations to individual requirements obviously being possible. Hydrogen in the form of fresh gas and circulating gas is compressed to overcome the pressure drop existing in the system. At the same time, but separate from each other, hydrogen and the starting material to be hydrogenated are fed to a heat exchanger. Here, the reactants are preheated by the reaction product and, if they are not already in the gaseous state, are at least partially vaporized, while the reaction product is cooled and higher-boiling components thereof condensed out.
The non-vaporized reactants can be converted into the gas phase in a subsequent heat exchanger, except for a small liquid portion which is withdrawn, and finally heated in a superheater to the reaction temperature and fed to the reactor. Here, the hydrogenation is carried out at substantially constant temperature, and the heat liberated during this hydrogenation is utilized for steam generation. The hot product passes into the above-mentioned heat exchanger and then into a condenser. A hydrogenation process proceeding approximately in the stages outlined above is described, for example, in Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 3rd Edition (1953), pp 778, 779 for the hydrogenation of crotonaldehyde to give butanol.
In practice, it is of essential importance to organize the heat balance of the overall process to achieve as high a degree of energy efficiency as possible. Furthermore, the activity of the catalyst should not be damaged by foreign substances (in particular, higher boiling materials) which are carried over into the hydrogenation reactor by the gaseous reactants. For this purpose it is necessary, inter alia, to separate the by-products from the product stream entering into the hydrogenation reactor as completely as possible.
It has been shown that relatively high pressure drops in the hydrogenation plant lead to a rise in the gas outlet temperature at the compressor and, as a consequence thereof, impair the heat transfer at the heat exchanger. Furthermore, although it is known that a horizontal arrangement of the heat exchanger improves the condensation of the reaction product, it also decisively reduces the heat transfer on the tube side of the heat exchanger.